Sawn lumber in standard dimensions is the major construction material used in framing homes and many commercial structures. The available old growth forests that once provided most of this lumber have now largely been cut. Most of the lumber produced today is from much smaller trees from natural second growth forests and, increasingly, from tree plantations. Intensively managed plantation forests stocked with genetically improved trees are now being harvested on cycles that vary from about 25 to 40 years in the pine region of the southeastern and south central United States and about 40 to 60 years in the Douglas-fir region of the Pacific Northwest. Similar short harvesting cycles are also being used in many other parts of the world where managed forests are important to the economy. Plantation thinnings, trees from 15 to 25 years old, are also a source of small saw logs.
Whereas old growth trees were typically between two to six feet in diameter at the base (0.6 m to 1.8 m), plantation trees are much smaller. Rarely are they more than two feet (0.6 m) at the base and usually they are considerably less than that. One might consider as an example a typical 35 year old North Carolina loblolly pine plantation tree on a good growing site. The site would have been initially planted to about 900 trees per hectare (400 per acre) and thinned to half that number by 15 years. A plot would often have been fertilized one or more times during its growth cycle, usually at ages 15, 20 and 25 years. At harvest the 35 year old tree would be about 40 cm (16 in) diameter at the base and 15 cm (6 in) at a height of 20 m (66 ft). Trees from the Douglas-fir region would normally be allowed to grow somewhat larger before harvest.
American construction lumber, so-called "dimension lumber", is nominally 2 inches (actually 11/2 inches (38 mm)) in thickness and varies in nominal 2 inch (51 mm) width increments from 31/2 inches to 111/4 inches (89 mm to 286 mm), measured at about 12% moisture content. Lengths typically begin at 8 feet (2.43 m) and increase in 2 foot (0.61 m) intervals up to 20 ft (6.10 m). Unfortunately, when using logs from plantation trees it is now no longer possible to produce the larger and/or longer sizes and strength grades in the same quantities as in the past.
There is another problem with plantation wood lumber that is not as generally recognized as are the tree size limitations. Typically, in plantation wood the average wood density is lower than old growth wood. This, in turn, affects strength and stiffness. Strength in flexure, otherwise termed modulus of rupture (MOR), and especially the stiffness measured as modulus of elasticity in flexure (MOE), may be lower and more variable than old growth wood. This is a problem for members used in a bending situation and it can be one for those members used in compression; e.g. longer wall studs. Typical of bending uses are floor joists, roof rafters, truss members, and headers over wide windows and doors, such as garage doors.
The problems noted above were outlined 20 years ago in a paper by A. Bendtsen Forest Products Journal 28 (10): 61-72 who noted the implications for construction lumber but offered no suggestions how to deal with them.
Since loblolly pine (Pinus taeda L.) and its closely related southern pines are particularly important timber species they will be used in the following discussion as a non-limiting example of coniferous trees in general. A frequently used unit related to density is specific gravity measured as oven dry weight/green volume. For loblolly pine, near the base of the tree specific gravity of the first several growth rings surrounding the pith will typically range around 0.38. By about age 20 the wood being formed near the bark at the same height will have a specific gravity of about 0.51-0.56. Density even of the outer mature wood portion of the tree varies longitudinally along the tree, being generally lower in the upper portions. Density of woods has been shown to correlate directly with stiffness, measured as modulus of elasticity in flexure. This variability has not been seriously taken into account in the manufacture of lumber products. Current sawmill procedures make no attempt to specifically deal with these inherent differences in density. The general assumption appears to have been that density variability was a factor which was not subject to any control.
Solid sawn wide dimension lumber is not without its own significant drawbacks. In particular, inconsistency in dry dimensions and strength properties and limited availability of long lengths are major deficiencies. Decrease in moisture content after installation causes shrinkage which is not consistent from piece to piece due to differences in grain orientation. This results in variability in dry width even though initial width was uniform. Particularly when the lumber is used as floor joists, inconsistent width from piece to piece results in poor conformation of sheathing or subfloor laid over the joists. This is a major contributor to the cause of annoying squeaks as people walk on the floor.
Lumber is graded visually by established rules that take into account many factors; i.e., knot size and placement, density, grain slope, manufacturing defects, etc. Any piece of lumber within a given grade is presumed to have some minimum stress rating. Unfortunately, the actual stress ratings of individual pieces within any one grade will vary considerably since the rules are established to ensure that the poorest piece will fall within grade.
Many approaches have been taken to engineer structural grade wood products to take the place of the larger and/or longer lumber sizes now in short supply. One successful approach is based on adhesively bonding a number of plies of rotary cut veneer. Unlike typical plywood products, the grain direction of all the plies is normally in the same direction. In one way of producing this product wide panels of appropriate thickness are ripped into pieces of standard dimension lumber width then finger jointed to the desired length. Other processes start with relatively narrower veneer sheets which can be butted end-to-end and continuously bonded to make units of almost any desired length, width, and thickness. The butt joints of adjoining plies are preferably staggered to prevent introducing points of weakness. This so-called laminated veneer lumber (LVL) has been in commercial production and use for a number of years, often as the tension members of trusses; e.g., as seen in Troutner, U.S. Pat. No. 3,813,842. It has the advantage that defects, particularly knots, do not run entirely through the piece as they do in sawn wood. This generally allows a higher stress rating for a LVL member of any given cross sectional dimensions. Other exemplary products of this type are described by Peter Koch, Beams from bolt-wood: a feasibility study, Forest Products Journal, 14: 497-500 (1964) and by E. L. Schaffer et al., Feasibility of producing a high yield laminated structural product, U.S.D.A. Forest Research Paper FPL 175 (1972).
Many combinations of veneer, solid sawn wood, and reconstituted wood such as engineered strandboard or flakeboard have also been explored for use as structural lumber products. Lambuth, in U.S. Pat. No. 4,355,754, shows a structural member in the form of an I-beam using a plywood web with solid sawn flange members. When used as a joist, this is presumably substitutable for sawn lumber of the same cross sectional dimensions. The web is friction fit and glued into tapered slots in the flange pieces. Other very similar constructions use composite wood strips such as oriented strandboard or flakeboard as the web member.
Barnes, in U.S. Pat. No. 5,096,765, notes the importance of stiffness (modulus of elasticity in flexure) (MOE) in lumber products. The product described uses splinters or strands of sliced veneer from 0.005-0.1 inch (0.13-2.5 mm) thick, at least 0.25 inches (6.4 mm) wide and at least 8 inches (203 mm) long. These must be free of any surface or internal damage and have their grain direction within 10.degree. of the longitudinal axis of the product. After addition of adhesive the product is pressed to have "an MOE equivalent to a composite wood product having a MOE of at least 2.3 mm psi [1.59.times.10.sup.7 kpa] at . . . a density of 35 lbs/cubic foot".
In the above patent the inventor refers to his earlier U.S. Pat. No. 4,061,819 which teaches that the strength of wood composite products is density dependent; i.e., ". . . the higher [the] density generally the higher the strength of the product for the same starting materials". The earlier patent describes a very similar lumber-like product to the above having a modulus of elasticity approaching or reaching the MOE of clear Douglas-fir at various densities. Products similar to those described in the Barnes patents are now commercially available. However, the very high adhesive usage they require has a significant negative impact on cost of the products. Also, the strandwood products have significantly higher density than sawn lumber and are heavier to handle and more expensive to ship.
Many other patents teach the manufacture of clear wood members by various combinations of sawing and edge, end, and/or face gluing. Exemplary of these are U.S. Pat. No. 1,594,889 to Loetscher, U.S. Pat. No. 1,638,262 to Neumann, U.S. Pat. No. 2,942,635 to Horne, U.S. Pat. No. 5,034,259 to Barker, and U.S. Pat. No. 5,050,653 to Brown. Other workers have explored surface densification for various purposes. Exemplary of these are U.S. Pat. No. 3,591,448 to Elmendorf and U.S. Pat. No. 4,355,754 to Lund et al.
Compressed wood products have been known for many years. One commercially available product is formed of a plurality of thin parallel grain veneer sheets that have been impregnated with a thermosetting resin prior to compression. This product is limited to specialty uses, principally kitchen and table knife handles. Walsh et al. in U.S. Pat. No. 1,465,383, describe a cross laminated compressed wood product useful for pulleys and similar items. Travis, in U.S. Pat. Nos. 4,136,722 and 4,199,632 shows a tool handle made of parallel laminated veneer sheets. The veneer sheets at the tool attachment end of the handle are interleaved with additional narrow veneer strips. The product is then compressed to uniform thickness so that the tool attachment end is of significantly higher density than the residual portion of the handle.
An earlier development by some of the present applicants, published as PCT Application WO 98/10157, describes selective placement of the denser wood from the trees along the edges of lumber products where it enhances stiffness and bending strength.
Most of the products noted above have not found significant success for one or more reasons. There are exceptions, however. Laminated veneer lumber and edge and end glued pieces reassembled to produce clear boards or for use as door cores have been in commercial use for many years. Composite I-beams similar to those described in the Lambuth patent are now also widely available. One such product family manufactured by Trus Joist MacMillan, Boise, Id., is typical of the products which appear to have become an industry standard.
The composite I-beams have found considerable acceptance in the building industry where long spans, consistent dimensions, and known and dependable strength properties are required. However, they are not without their drawbacks. Their performance under common residential dynamic loads is not as good as solid sawn construction, due primarily to a lack of mass. As a result most builders use I-joists at a shorter than suggested span or at a reduced spacing. They cannot entirely be used as a replacement for sawn lumber. For example, they need reinforcing blocking to fill out the sides of the web to full width at many loading points. Their cross section essentially prevents side nailing and they present a major problem in attaching other members to the sides. Also, since the flange portions of the I-joist provides most of the stiffness it cannot be notched as is commonly done with solid sawn lumber. The nature of the geometry increases shear forces in the web member to higher values than are found in solid products of rectangular cross section.
It is notable in view of the highly heterogeneous nature of the smaller trees now available that the art has not more seriously heretofore addressed the problem of producing strong members of uniform and dependable properties from smaller plantation trees. The present invention overcomes the noted deficiencies in solid sawn lumber and composite I-beams. In addition, it results in a much higher utilization of the tree into useful lumber products.